This invention relates to navigation systems in general and more particularly to an improved method for carrying out the rapid-alignment of an aircraft inertial platform using a low-cost aircraft type shipboard reference system.
Carrier based aircraft, when taking off from the carrier must have their inertial platforms properly aligned in azimuth and vertically, to reflect the existing state vector as they are on the carrier. Typically, such has been carried out by using a shipboard inertial navigation system of extremely high accuracy. Such an inertial system is lightly damped by the EM log which supresses the Schuler oscillations caused by random component drift. Once aligned, such a system provides very accurate velocity and position outputs over a long period of time. Position fixes are used occasionally to update the system. Such a system provides alignment outputs (North and East velocity) to an aircraft which then uses this data for rapid alignment of its own inertial platform. To carry out such rapid alignment Kalman filtering is typically used in the aircraft. Such a system works quite well. However, the highly accurate ship's inertial navigation system used has a cost in the range of two million dollars. In view of this, the desirability of being able to carry out such an alignment using an aircraft type inertial platform which is of lower accuracy and cost become evident. However, if such a platform is used it must provide sufficient accuracy to properly align the aircraft platform.
Previous attempts at using aircraft type inertial platforms as shipboard references have not been particularly successful.
In such systems the velocity and azimuth accuracy diverges over a fairly short period of time if realignment is not carried out. Even with good initial alignment at dockside, the velocity accuracy during the first several hours will typically be in the order of one foot/second. The platform must be realigned at sea at regular time intervals using information from an EM log and/or electronic positioning equipment such as a Loran or Omega system. Initial alignment of such a platform at sea would require at least one hour and probably longer. As a result, the use of such a system requires either alternating between alignment and navigation modes or remaining permanently in a hybrid alignment navigation mode. The system may also be operated by remaining in an hybrid mode with regular imputs to a ship's Kalman filter being provided from the inertial platform, an EM log and a device such as an Omega or a Loran. In this way, the Kalman filter continually provides an accurate estimate of the desired state vector quantities. In point of fact, through such an implementation a velocity error on the order of one foot/second or better can be obtained. However, when an output of this nature is provided from ship's Kalman filter as an input to an aircraft platform system an accurate azimuth reference is not obtained unless an excessive alignment time exists. This is demonstrated by FIG. 1 which illustrates the error in recovered heading in the aircraft as a function of the correlation time of correlated noise where correlation time is the reciprical of the noise band width. As this figure illustrates, with a short alignment time of 300 seconds and only 0.25 ft/second of reference noise, a half degree error in heading or azimuth can result. Only by extending the alignment time to a much longer period such as a length of 1,000 seconds will the Kalman filtering result in the desired low error. Because of this effect of correlated noise on azimuth accuracy, this approach has been considered to be unworkable. There have been attempts to overcome these deficiencies mostly by modeling, in the aircraft's Kalman filter, the reference velocity correlated noise. Clearly such as increased complexity in the aircraft is not desirable.
Thus, it can be seen that there is a need for a method of employing a low cost aircraft inertial platform as a shipboard reference for aligning aircraft platforms. Aircraft inertial systems have a cost which is at least an order of magnitude less than the cost of a highly accurate shipboard inertial system, i.e., the difference between a hundred thousand dollars and two million dollars. In view of this, the development of such a system can result in considerable cost savings for the user.